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Date: Fri, 26 Jan 2018 23:55:56 -0800
From: Dan Williams <>
Cc:,,,, Ingo Molnar <>,
 Andy Lutomirski <>, "H. Peter Anvin" <>,,
Subject: [PATCH v5 08/12] x86: sanitize sycall table de-references under

The syscall table base is a user controlled function pointer in kernel
space. Use 'array_idx' to prevent any out of bounds speculation. While
retpoline prevents speculating into a userspace directed target it does
not stop the pointer de-reference, the concern is leaking memory
relative to the syscall table base, by observing instruction cache

Reported-by: Linus Torvalds <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: "H. Peter Anvin" <>
Cc: Andy Lutomirski <>
Signed-off-by: Dan Williams <>
 arch/x86/entry/common.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 03505ffbe1b6..f78bf8bfdfae 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -21,6 +21,7 @@
 #include <linux/export.h>
 #include <linux/context_tracking.h>
 #include <linux/user-return-notifier.h>
+#include <linux/nospec.h>
 #include <linux/uprobes.h>
 #include <linux/livepatch.h>
 #include <linux/syscalls.h>
@@ -284,6 +285,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
 	 * regs->orig_ax, which changes the behavior of some syscalls.
 	if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) {
+		nr = array_idx(nr & __SYSCALL_MASK, NR_syscalls);
 		regs->ax = sys_call_table[nr & __SYSCALL_MASK](
 			regs->di, regs->si, regs->dx,
 			regs->r10, regs->r8, regs->r9);
@@ -320,6 +322,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 	if (likely(nr < IA32_NR_syscalls)) {
+		nr = array_idx(nr, IA32_NR_syscalls);
 		 * It's possible that a 32-bit syscall implementation
 		 * takes a 64-bit parameter but nonetheless assumes that

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